The beauty and clearness of the dynamical theory, which asserts heat and light to be modes of motion, is at present obscured by two clouds.

From a 1900, April 27, Royal Institution lecture. “Lord Kelvin, Nineteenth Century Clouds over the Dynamical Theory of Heat and Light”, reproduced in Notices of the Proceedings at the Meetings of the Members of the Royal Institution of Great Britain with Abstracts of the Discourses, Volume 16, p. 363–397 and Philosophical Magazine, Sixth Series, 2, 1–40 (1901).